Teaching Earth Sciences - Earth Science Teachers' Association

the results are extremely striking. It has been estimated that
Arctic (summer) temperatures increased from around 18 ºC
to over 23 ºC during this event (Sluijs et al., 2006; Weijers
et al., 2007b). These extremely high values indicate that
the Arctic would probably have been ice-free year round.
The temperatures are so high that climate models find
them difficult to replicate, indicating that greenhouse gas
concentrations must have been much higher than those
of the present day and probably operated in conjunction
with other feedback mechanisms to result in such high
early Palaeogene Arctic temperatures (Sluijs et al., 2006).
Sedimentological evidence from the Vest Spitsbergen
sections indicates that at the height of the warming
event sea level reached its maximum level (possibly due to
thermal expansion), resulting in a marine incursion, water
column stratification, and lowered bottom-water oxygen
levels. Stratification was further enhanced by surface water
freshening which was caused by an increase in terrestrial
runoff – another consequence of the global warming
– indicated by huge abundances of low salinity plankton. A
picture emerges of an ice-free Arctic PETM characterised by
high eustatic sea levels, and stratified, deoxygenated water
masses supporting unusual, high-dominance plankton
assemblages.
The Azolla Event (~50 million years ago)
The dramatically enhanced high northern latitude
precipitation indicated by the Arctic PETM results
described above corroborates fully coupled palaeoclimate
simulations of the early Palaeogene greenhouse world.
However, the ACEX core provided some even more
extraordinary evidence for such conditions in the form of
massive abundances of the reproductive structures of the
free-floating fern Azolla found in middle Eocene Arctic
sediments (Brinkhuis et al., 2006). Azolla was clearly
flourishing in the restricted Arctic Basin surface waters
(Figure 3). These surface waters are beieved to have been
freshened by increased terrestrial runoff, as the only
other microfossil present in these sediments are also of
freshwater affinity: diatoms and chrysophyte cysts. These
freshened surface waters existed in the Arctic for a period
of some 800,000 years. Lower concentrations of Azolla
are also found in marine sediments from the Nordic seas
surrounding the Arctic, and it has been suggested that
these occurrences may represent spill-overs of freshwater
from the Arctic Ocean which transported the fern
remains further south (Brinkhuis et al., 2006; Figure 3).
An increase in salinity and sea surface temperatures seem
to have terminated the Azolla blooms in the Arctic Basin.
However, it has been postulated that the high Palaeogene
atmospheric carbon dioxide levels could have been reduced
by the Azolla blooms, CO 2
being converted into organic
carbon that was then locked into Arctic bottom sediments,
so high are the concentrations of Azolla and other organic
material in these middle Eocene sediments.
The Eocene-Oligocene greenhouse-icehouse transition
(~33.5 million years ago)
The boundary between the Eocene and Oligocene epochs
(~33.5 million years ago) is a critical phase in Earth history.
An array of geological records, including a rapid two-step
shift in benthic foraminiferal oxygen isotopes (the
Oi-1 glaciation event), and climate modelling experiments
indicate a profound shift in global climate, from a world
largely free of polar ice caps to one in which Antarctic ice
sheets approached their modern size (DeConto & Pollard,
2003). However, until recently there was very little known
of the early glaciation history in the high northern latitudes
(Zachos et al., 2001). New analyses of ODP Site 913B has
yielded evidence for extensive ice-rafted debris, including
dropstones up to 7cm in diameter, in late Eocene to early
Oligocene sediments from the Norwegian–Greenland Sea
that were deposited between about 38 and 30 million
years ago (Eldrett et al., 2007). These dropstones are likely
to have been rafted to the site of deposition by glacial ice
(shed from nearby East Greenland) rather than sea ice,
something corroborated by grain surface structures. These
data suggest the existence of isolated glaciers on Greenland
about 20 million years earlier than had been supposed
previously (Eldrett et al., 2007).
Figure 3 Palaeogeographic reconstruction of the Arctic Ocean during the Azolla
event, showing positions of the ACEX site (ODP Site 302-4a) in the Arctic and ODP
Site 913B in the Norwegian-Greenland Sea. Large white arrows indicate suggested
routes of freshwater spill-overs carrying Azolla into the Nordic seas (based on
Brinkhuis et al., 2006).
Other studies have now corroborated the sedimentological
evidence for climatic deterioration in the Arctic prior to the
Eocene-Oligocene boundary (EOB). Our studies using the
CBT/MBT proxy indicates a mean annual air temperature
(MAAT) in the late Eocene of ~13–15 °C, with a gradual
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